1. Field of the Invention
The present invention relates to a system for constructing interior building walls that will withstand both seismic activity and fire.
2. Description of the Prior Art
Seismic and fire resistance has become of increasing concern in building construction. In the construction of buildings the framework for the walls of a building is formed of horizontal sill members at the floor, at the ends of which vertical corner posts support horizontal beams at the ceiling level. Between the corner posts there are upright supports, called studs, laterally spaced, usually at uniform intervals, to provide the necessary interior structural support for the wall.
Historically, the framework of a building wall was formed entirely of wooden members, including wooden studs. In recent years, however, the use of metal studs has gained increased acceptance, especially in the construction of commercial buildings, such as office buildings, schools, and hospitals. It has been found that metal studs can be employed to advantage, since a suitable metal, such as twenty-gauge galvanized steel, is stronger than wood, and therefore offers greater resistance to seismic forces. Moreover, metal studs will not burn as wood does, will not rot, and are not subject to damage by pests, such as termites. The use of metal studs also reduces the depletion of hardwood forests. Furthermore, metal studs are now economically competitive with wooden studs in the building construction industry.
While wooden studs are formed of solid wood, typically having nominal cross section dimensions of two inches by four inches, the much greater structural strength of metal allows building studs to be employed which are not solid, but rather are hollow and have a channel or "C-shaped" cross section. To conform to the architectural plans and building materials which have been developed over the years based on the use of wooden studs having specific cross sectional dimensions, commercially available metal studs are constructed with the same outer dimensions in which wooden studs have been manufactured for many years. Specifically, metal studs are typically formed of sheet metal bent to encompass a cross sectional area having nominal dimensions of two inches by four inches.
For ease of fabrication the metal studs are formed of sheet metal bent into a generally "U-shaped" cross section and in which a relatively broad central web is flanked by a pair of narrower sides that are bent at right angles to the web or base. The web typically has a uniform nominal width of either four inches or three and one half inches, and the sides of the U-shaped stud typically extend a nominal distance of two inches from the web. To enhance structural rigidity the edges of the sides of the metal stud are normally bent over into a plane parallel to and spaced from the plane of the web. These turned over edges of the side walls thereby form marginal lips which are typically one quarter to one half an inch in width. The finished stud therefore has a generally "C-shaped" cross section.
The overhead beams that extend along the tops of the studs in interior building wall construction have a U-shaped configuration. They are each formed with a horizontally disposed web from which a pair of side walls depend vertically on opposite sides of the web. The side walls embrace the sides of the vertical studs so that the upper extremities of the studs extend perpendicular into the concave, downwardly facing channel formed by the overhead beam. The spacing of the studs along the length of the beam is typically either sixteen or twenty-four inches. In a nonload-bearing wall the web of the beam is secured to the ceiling above by screws that extend vertically upwardly through the web of the beam and into the structure of the ceiling.
One problem which occurs in any building during an earthquake is that the seismic ground motion produced by an earthquake creates both horizontal and vertical undulations in the building. The elongated, vertical lengths of metal studs in building wall construction render these studs limber enough to flex sufficiently in a lateral direction and thereby resist inelastic deformation during an earthquake. However, vertical undulations that vary the distance between the floor and ceiling in a room during an earthquake are more likely to destroy, or at least damage the integrity, of the structural joints between vertical metal studs and horizontal sill and overhead beam members between which the studs extend in a building.
To alleviate this problem a seismic and fire resistant wall structure and method was devised. This system is described in U.S. Pat. No. 5,127,203. According to this system the overhead beam that extends across the top of the upright studs is provided with vertically elongated slots which are longitudinally spaced at intervals to accommodate the positions of studs within a vertical, nonload-bearing wall. Fasteners extend through the vertically elongated slots in the overhead beams and into the sides of the studs. The fasteners, typically sheet metal screws, are tight enough to provide lateral stability at the joints between the studs and the overhead beam, but are not so tight as to totally prevent relative vertical motion therebetween.
As vertical undulations from an earthquake are transmitted through the structural components of a nonload-bearing wall, the elongated, vertical slots through which the studs fasteners extend permit limited, vertical, oscillatory motion to occur between the upper extremities of the studs and the overhead beams of the nonload-bearing walls. As a result, the stud fasteners maintain structural integrity so that the wall remains undamaged and does not require repair following an earthquake.
Another consideration which also is of great concern in building construction is the resistance to fire. In the conventional construction of nonload-bearing walls employing metal studs, sills, and overhead beams, the metal members, of course, are fire resistant. Furthermore, fire-resistant wallboard is attached to the opposing side faces of the studs and extends between the floor and the channel-shaped beam member. The nonload-bearing, interior walls thereby form a fire block and create a vertical barrier to the spread of a fire. However, one problem which has not heretofore been adequately dealt with is the matter of fire resistance at the interfaces between the overhead beams atop the metal studs and the ceiling to which the beams are connected.
In a typical building construction a ceiling is formed by galvanized steel, fluted decking atop which a layer of concrete is poured to form the floor above. The fluted steel decking may, for example, be eighteen gauge galvanized steel. The flutes, or concave, downwardly facing channels defined in the underside of the decking, are typically about three inches deep and either four or six inches wide. Interior walls often pass transversely across the flutes, and the beams at the tops of such walls are attached to the underside of the decking where the decking projects downwardly between the hollow flutes. Openings having cross-sectional areas equal to the areas of the flutes are thereby formed above the beams that are located at the top of nonload-bearing, interior walls. These openings form lateral tunnels across the tops of the walls through which fire can travel unless blocked.
To prevent the spread of fire through the flutes formed by the decking above nonload-bearing, interior walls, fire-resistant insulation is packed in the flute openings where these tunnels pass across the top of the walls. A typical, conventional fire insulation material of this type is Monokote MK-6/CBF. This fire-resistant insulation is applied by spraying it into the flute openings from each side of the wall. As long as the insulation remains in the flute openings, these tunnels are blocked and prevent the spread of fire therethrough. However, when a fire is burning within a building, it generates a considerable amount of smoke which is heated and expands. The smoke creates a substantial pressure within a room where a fire is burning. It has been discovered that the pressure of smoke from a fire burning within a room literally blasts the fire insulation out of the flute openings atop the wall. When this occurs the fire can thereupon spread to an adjacent room over the top of the wall through the flute openings.
According to present building construction practice fire insulation is held within the tunnel cavities defined by the flutes of the decking by hand cutting the upper edges of wall panels to follow the corrugations of the decking. The wallboard panels forming the sides of the nonload-bearing walls provide a series of projections that block the flute tunnels from the opposite sides of the wall and thereby hold the insulation in place. However, this method of holding the insulation in position is extremely time consuming, laborious, and expensive.
Hand cutting of the upper region of the wall to follow the convolutions of the corrugated, fluted decking is extremely labor intensive. This adds significantly to the cost of construction of the wall. Moreover, even if a template is used the hand cuts result in significant gaps remaining which must then be caulked. The process of caulking is also an extremely laborious, labor intensive process, particularly when it is necessary to follow the convolutions of the underside of the fluted decking. Moreover, conventional caulking is not seismic resistant. That is, even if the caulking originally provides an effective barrier to air currents, if the building structure subsequently is subjected to seismic activity, the caulking crumbles and gaps that allow the passage of air currents are opened. When this occurs the wall no longer offers its original resistance to the spread of fire. As a result, it has not heretofore been possible to provide both seismic resistance and fire resistance in building walls that will meet the stringent building codes applicable to structures such as schools and hospitals.
A principal object of the present invention is to provide an interior building wall construction that will meet both stringent seismic and fire resistance code standards. For example, the UL (Underwriter's Laboratory) Standard 2079 requires that joints in metal stud framing withstand twenty cycles of a one-half inch linear movement of the structures joined together. The wall system of the present invention successfully withstands cycling of one hundred cycles of one full inch of linear movement. Also, UL Specification 2079 additionally requires the joints of a wall to remain fire resistant for a full hour, in the case of some interior walls, and for two hours in the case of others. After subjecting the wall to fire, the wall joint and the insulation in the cavities of the flutes atop the wall must withstand the pressure applied by a stream of water directed thereon from a firehose to simulate the pressure produced within a building due to fire. In conventional building construction systems the blast from the fire hose readily dislodges the insulation from the cavities created by the flutes above the wall beam unless the wallboard has been cut to follow the undulations of the ceiling flutes and thereby protect the insulation.
In testing building wall systems for fire resistance, the joints are expanded to the maximum joint opening width for which the system is intended to function. Thus, it is evident that design features that tend to enhance seismic resistance tend to reduce fire resistance. That is, if there is considerable play in the joints between upright metal studs and overhead metal beams to which the studs are attached, openings are created which reduce resistance to the passage of fire. One the other hand, if joints are closed and locked immovably together, they are likely to fail when subjected to seismic activity. Thus it has heretofore not been possible to provide an interior building wall construction system which meets both the maximum standards for fire resistance and the maximum standards for resistance to seismic movement as well. However, the system of the present invention easily surpasses both fire and seismic resistance code specifications that are currently in use.